15% Difference between Pix4d volumes and manual survey

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Hello,

I am using a trial version of Pix4D and a new Mavic 2 Pro drone to survey stockpiles. So far, as described in the subject line, the results are unacceptable.

I have flown 2 surveys and in both cases the difference between the drone volumes and manually measured volumes is over 10%. The manual method is considered accurate, so I need to either improve my drone surveying/processing technique or convince people that the manual method wasn’t that good after all!

The surveys were actually flown with DroneDeploy, as Pix4DCapture doesn’t work for me (another post some other day). I used the Overlap Optimizer tool to ensure adequate coverage. The parameters were 80 m high, 75% front overlap & 65% side overlap for the first survey resulting in 232 images,  and  60 m, 76 % & 73 % for the second, with 178 images. The stockpiles are 10 to 15 m high.

I also surveyed some of the piles manually using a Topcon/Magnet system. Sadly these are done with local coordinates so it is impossible to merge the two systems for a better comparison.

The Pix4d  processing is completely default. I am using the triangulated option when defining the base of the piles. I picked the boundaries from the volumes view. I found it useful to rotate the perspective as I went to visualize the edge of the pile.

For the first survey I obtained volumes of 4850, 1907 & 3574 m3 manually and 5544, 2260 & 3856 m3 with the drone, which corresponds to differences of 14%, 18% and 8%. The results of the second survey were similar, but the manual survey was done by someone else.

I can understand and expect the drone survey to be somewhat higher because of increased resolution but not this much.

My questions are…

  1. I assume other people have done similar comparisons… what is a typical difference?
  2. Its hard to ensure I’m picking the boundary correctly. It is sometimes quite difficult to spot  the edge of the pile. Any tips, pointers, docs or videos I should watch?
  3. Are the default processing parameters suitable?
  4. I would find it a lot easier to pick the boundary points in the orthophoto which is much more detailed. Any way of doing that?
  5. I’ve exported a boundary to a shape file, imported to qgis and then reexported and the volume went from 2260 to 2166, which is kind of amazing. A detailed description of how to do that process correctly would be very useful.

cheers,

Marc

2

Hi Marc,

We have noticed that sometimes with the Mavic 2 Pro there are some calibration issues if the default parameters are used. We instead recommend using the All prior option for the Internal Camera parameters optimization, if the camera optimization does not give good results.

Could you share with us the quality report of the project?

You can follow this post of our Community on Mavic 2 Pro issues and particularly read this comment by Daniel.

Also, the Error estimation in volume calculation is strictly linked to the GSD of the project and the bigger the area is, the bigger the error will be. You haven’t mentioned any GCPs. Are you using some to increase the Accuracy of Pix4D outputs?

Relatively to the vertices picking, you can have a look at this other post of the Community. Is this what you were looking for? I will consider adding the suggestion you pointed out in point 5 of your list.

Finally, concerning the data acquired with the Topcon/Magnet system, you should first convert the coordinates to the output coordinate system in Pix4D or if you want to work in your local system, Pix4D can compute the transformation parameters to your local system with the site calibration functionality. I would say that would be the best option.

Cheers

3

Hi Alice

Thanks for your response. It’s too bad it took a week because my trial has expired and I can’t see if it worked!

 

4

Hi Marc, 

an additional trial has already been activated under your profile. You still have 7 days to discover what Pix4Dmapper can do. I hope you can try this out and that the software suits your needs. Let me know if the suggestions helped.

5

Alice and Marc,

Alice’s solution worked!

I had a very similar issue with the Mavic 2 Pro. I flew the same piles with a Phantom 4 Pro previous to the Mavic 2 Pro and knew that in reality the stockpiles were depleted and that nothing was added to them over the period of one month. However, according to Pix4D, the stockpiles I volumes had actually increased over this month and I noticed that the camera parameter error was occurring. I tried adjusting different camera parameters (reading from exif and optimizing camera parameters). I finally found this thread and Alice’s suggestion for putting the “internal parameters optimization” setting to “all prior” worked. Now the results show a 0% difference in the results. That green check-mark has never looked so good!

6

Hey Mark,

thanks for your feedback! Happy to read your comment :slight_smile: